For our purposes we'll use this "application". It has a single function that has input, output and some "business logic". (Yeah, I know, adding two numbers is probably not such a huge business these days.)

examples/python/mock-input/app.py

def main():
    # ...
    x = input("First: ")
    y = input("Second: ")
    # ...
    z = int(x) + int(y)
    # ...
    print("The result is {}".format(z))



if __name__ == '__main__':
    main()

We then have a test like this:

examples/python/mock-input/test_app.py

import app

def test_app():
    input_values = [2, 3]
    output = []

    def mock_input(s):
        output.append(s)
        return input_values.pop(0)
    app.input = mock_input
    app.print = lambda s : output.append(s)

    app.main()

    assert output == [
        'First: ',
        'Second: ', 
        'The result is 5',
    ]

In this code we have two lines in which we replace the input and print functions of the app object. The overriding of print seems to be the simpler here.

Every time the code in app calls print our lambda function will append the value to the list called output we have declared earlier. It won't print anything to the screen. (In case you are not familiar with lambda it creates an anonymous function on the fly.)

Overriding the input function is similar, but that required two statements, so we could not do it in a lambda. Hence we created a function called mock_input (a totally arbitrary name) and then assigned that function to the app.input. That means every time input is called inside the app object, Python will call our mock_input function instead of the built-in input function.

Normally the input function of Python 3 does 2 things: prints the received string to the screen and then collects any text typed in on the keyboard. Our function will also do two things: Append the received string to the list called output just as the lambda function of print does and then take the first value from the list called input_values and return that.

This allows us to prepare a list of answers we would like to give to the application and this function will help us pretend we have actually typed those in when the application asked for it.

Once we prepared all this we call the main function of the application: app.main().

This will call the fake input function and the fake print function, but the rest of the code won't know about the change. So the rest, the business logic, we work as it should.

Then we can assert whether the values collected in the output list are the same values as we expected them.

We can run the test by typing pytest test_app.py in the directory where we have both of these files. (In our case that is the examples/python/mock-input/ directory.

The above works, but we might have better solutions:

capsys

capsys is one of the built-in fixtures Pytest provides. It helps us capture everything that goes to the standard output and the standard error during the execution of a test.

In order to use it we need to include the parameter capsys in the list of parameters our test function expects. (In our case this is the only parameter.) Seeing that the function expects the capsys object Pytest will call our function passing in the capsys object (this technique is called dependency injection.) Pytest will also set up everything necessary to capture both the output and error stream.

That means we don't need to (and we really should not) override the print function and in the mock_input function we don't save the parameter string in an output list. Actually, because our mock_input function only has one statement in it, we could have converted it to a lambda. But I digress.

Once we have our setup we call app.main() just as in the previous case. Then we can call the readouterr method of the capsys object that will return two strings. Whatever was printed to the standard output and whatever was printed to the standard error since the beginning of the test function.

We can compare these two to the expected strings.

examples/python/mock-input/test_app_capsys.py

import app

def test_app(capsys):
    input_values = [2, 3]

    def mock_input(s):
        return input_values.pop(0)
    app.input = mock_input

    app.main()

    out, err = capsys.readouterr()

    assert out == 'The result is 5\n'
    assert err == ''

Using capsys has the advantage that it is a built-in tool and that it can be used to check both the correct output and that nothing was printed to the error channel.

In other cases we might use it to make sure the expected error message was printed to the error channel.

We should also notice that in this solution we don't capture the output that should have been generated by the input function as that part was swallowed by our mocking function. This might be clearer or more confusing to you depending on your expectations.

We also had to include the new-line \n in our expected string.

Try: pytest test_app_capsys.py

capsys capturing the prompts as well

If you'd like to include the prompt strings in the captured output, it can also be easily done. You just need to print them in the mock_input function.

examples/python/mock-input/test_app_capsys_print.py

import app

def test_app(capsys):
    input_values = [2, 3]

    def mock_input(s):
        print(s, end='')
        return input_values.pop(0)
    app.input = mock_input

    app.main()

    out, err = capsys.readouterr()

    assert out == "".join([
        'First: ',
        'Second: ', 
        'The result is 5\n',
    ])

    assert err == ''

Then we also need to include those strings in the expected output.

(We used end = '' in the print function inside the mock_input function as that's how input prints the prompt string.)

Try: pytest test_app_capsys_print.py

Warning

If you have all the 3 test files and the application in the same directory and you run pytest it will run all 3 test files and then some of them will fail. This is because they run in the same process and we override parts of the app in different ways in the different test files. In a real application you would stick to one of the above solution for all of your test files and then this problem would not happen.